Poly(vinyl alcohol) has very low gas permeability making it especially useful as a gas barrier material. However, due in part to its high polarity, its melting point is close to its decomposition temperature which can be problematic in coating applications. In addition, poly(vinyl alcohol) polymers absorb moisture. One approach to alleviate these problems has been to introduce hydrocarbon units into the chain of the poly(vinyl alcohol) polymer to form copolymers.
Graft copolymerization of poly(vinyl alcohol) is a known method of changing the composition of poly(vinyl alcohol) in order to change the properties. For example Polyvinyl Alcohol-Developments, edited by C. A. Finch, Wiley, 1992, pages 449-453 contains information on the graft polymerization of vinyl acetate with poly(vinyl alcohol). Polyvinyl Alcohol, edited by C. A. Finch, Wiley, 1973, pages 438-442, reviews the use of ceric complexes in forming grafted copolymers of poly(vinyl aclohol) and several ethylenically unsaturated monomers, such as, methyl acrylate, methyl methacrylate, acrylonitrile, acrylamide, and styrene.
Little is known about the polymerization, especially graft polymerization, of alkenes, and specifically ethylene, in a poly(vinyl alcohol) solution.
U.S. Pat. No. 3,926,934 (Lasswell, et al., 1975) discloses the production of polyethylene powders by high pressure polymerization of ethylene employing a water-soluble initiator, a controlled concentration of water, a dispersing agent, such as polyvinyl alcohol, and rapidly venting the polymerization zone or the polymerization effluent mixture withdrawn from the polymerization zone.
JP 51-73033 (1976) discloses wood adhesives which contain ethylenically unsaturated compounds or conjugated dienes graft-polymerized with poly(vinyl alcohol). Examples of ethylenically unsaturated compounds include styrene, acrylic and methacrylic acid and esters, unsaturated amides such as methacrylamide, and olefins, such as isobutylene. Butadiene and isoprene are examples of conjugated dienes. It is reported that a hydrophilic grafted polymer is formed in which the monomers are 0.2 to 5 parts by weight, per part by weight of poly(vinyl alcohol).
JP 64-45876 (1989) discloses the manufacture of a spray sizing agent consisting of a graft copolymer of poly(vinyl aclohol) and xcex1, xcex2-monoethylenic monomers such that the ratio of poly(vinyl alcohol) to monomer is 100:5-100. Examples of xcex1, xcex2- monoethylenic monomers include acrylates, methacrylates, vinyl acetate, and vinyl versitate. Other unsaturated monomers which reportedly can be combined with the above xcex1, xcex2-monoethylenic monomers include styrene, acrylamide, vinyl chloride, and ethylene.
JP11-152316 (1999) discloses alkylated polyvinyl alcohol in which ethylene is added to the polyvinyl alcohol side chain. The addition reaction is carried out in an aqueous solution of poly(vinyl alcohol) at a pressure of 10 to 80 kg/cm2 (1080 to 7948 kPa) using a radical initiator at levels of 0.1 to 100 wt %, preferably 1 to 50 wt %. In the examples, a 2.35 wt % aqueous solution of poly(vinyl alcohol) (PVA 217 and PVA 117; DPn=1700; hydrolysis of 88 mol % and 98 mol %, respectively; Kuraray Co.) was used to form alkylated polyvinyl alcohol. In the examples, the mean number of carbons in the grafted alkyl group was 12.6 to 13.8.
Poly(vinyl alcohol)-g-polyethylene copolymers have typically been made by grafting polyethylene to poly(vinyl alcohol). Examples of grafting methods are described below:
JP 58040323 (abstract) discloses the graft polymerization of poly(vinyl alcohol) onto HDPE films using xcex3 irradiation.
U.S. Pat. No. 4,575,532 (Schmukler et al., 1986) discloses a polyvinyl alcohol alloy which is prepared by reacting polyvinyl alcohol with a polyolefin which incorporates functional groups reactive with the hydroxyl groups in polyvinyl alcohol. Examples of suitable polyolefins are polyethylene, ethylene copolymers, and polypropylene.
U.S. Pat. No. 4,937,284 (Bergstrxc3x6m, 1990) discloses the manufacture of olefin/vinyl alcohol block copolymers by joining polyvinyl alcohol to a polyolefin, such as low density polyethylene, low pressure polyethylene, high density polyethylene, and polypropylene, with the aid of silane.
Watanabe, T., et. al. (Polym. Prepr., 34(1), 1993, pp. 912-913) (abstract) discloses the graft modification of poly(vinyl alcohol) with succinic anhydride modified LDPE.
Poly(vinyl alcohol)-g-polyethylene graft emulsion copolymers in which ethylene is graft copolymerized to poly(vinyl alcohol) in an aqueous solution to form graft emulsion copolymers containing 50 wt % or more polyethylene have not been reported.
This invention is directed to aqueous based grafted copolymers comprising a polymer having a plurality of hydroxyl groups, preferably a vinyl alcohol homo- or copolymer, grafted with polyethylene, and optionally another polymerized ethylenically unsaturated monomer. The graft emulsion copolymers can contain 50 to 95 wt % polyethylene, 5 to 50 wt % hydroxyl-containing polymer, and up to 45 wt % of another polymerized ethylenically unsaturated monomer. Some homo- or copolymers of ethylene can also be present in the graft emulsion copolymer product.
The graft emulsion copolymers are prepared by polymerizing ethylene and, optionally another ethylenically unsaturated monomer, in an aqueous solution containing at least 5 wt %, preferably at least 10 wt %, of a polymer having a plurality of pendent hydroxyl groups, preferably poly(vinyl alcohol). The poly(vinyl alcohol) can have a degree of polymerization (DPn) of 50 to 2200, preferably 100 to 1000; and most preferably 100 to 300. A high solids, i.e., 10 to 65 wt %, preferably 30 to 60 wt %, graft emulsion copolymer product is formed.
The graft emulsion copolymer of this invention can be useful for films and provide unique performance features in a variety of applications, such as temperature sensitive adhesives, paper coatings, nonwoven coatings, binders for nonwovens, gas barrier films, and emulsion polymerization applications. The addition of a polyethylene to poly(vinyl alcohol) results in improvement of the hydrophobic properties, enhancement of oxygen barrier properties, improvement of tensile, and improved ease of film formation.
The graft emulsion copolymers of this invention can be prepared by free radical initiated polymerization of ethylene, and, optionally, another ethylenically unsaturated monomer, in a solution of at least 5 wt %, preferably at least 10 wt %, of a polymer containing a plurality of pendent hydroxyl groups. The hydroxyl-containing polymer is preferably poly(vinyl alcohol).
Examples of other ethylenically unsaturated monomers are straight chain or branched alkenes, such propylene, butylene, isobutylene, pentene, and the like, wherein the number of carbons in the alkene ranges from 3 to 12, vinyl acetate, vinyl versatate, or other vinyl esters, alkyl acrylates, alkyl methacrylates, hydroxyalkyl acrylates, hydroxyalkyl methacrylates, acrylamides, methacrylamides, N-methylolacrylamide, and combinations thereof; wherein alkyl is C1 to C6 alkyl.
Polymers having a plurality of pendent hydroxyl groups can be formed by the polymerization of vinyl or acrylic esters in which the ester unit contains from 1 to 4 carbon atoms. The pendent ester groups, when hydrolyzed, form polymers containing pendent hydroxyl groups. A preferred class of polymers having a plurality of pendent hydroxyl groups are based upon hydrolyzed vinyl acetate polymers wherein vinyl acetate is polymerized as a homopolymer or in conjunction with other monomers to form copolymers and are known as poly(vinyl alcohol) or vinyl alcohol copolymers. The amount of vinyl alcohol should be sufficient to enable free radical graft polymerization in an aqueous solution; i.e., render the polymer at least partially soluble in an aqueous medium. Typically the vinyl ester, specifically vinyl acetate, will comprise from 60 to 100% of the copolymer, preferably at least 90 mol percent of the polymer. The poly(vinyl alcohol) used in this invention, generally, has a weight average molecular weight (MW) ranging from about 4,000 to 186,000, preferably 8,000 to 40,000; and most preferably, 8,000 to 23,000. Alternatively, the poly(vinyl alcohol) can have a degree of polymerization (DPn) of from 50 to 2200, preferably 100 to 500; most preferably 100 to 300. Poly(vinyl alcohol) is made commercially by the hydrolysis of poly(vinyl acetate) and typically has a hydrolysis level ranging from about 85 to greater than 99 mol %. For this invention, the level of hydrolysis can range from 50 to 100 mol %, preferably 85 to 98 mol %, and most preferably 86 to 92 mol %. Mixed poly(vinyl alcohol) grades in which the poly(vinyl alcohol) varies in molecular weight and hydrolysis level, can also be employed in the present invention. It is desirable that a mixture contain predominantly low to ultra-low molecular weight poly(vinyl alcohol), in order to maximize the solids levels and produce a graft emulsion copolymer with a high amount of polyethylene. By using low molecular weight poly(vinyl alcohol), better coalescence, favorable solids-viscosity relationships, and high overal product solids can be achieved with the graft emulsion copolymer.
The free radical graft copolymerization reaction is conducted in an aqueous solution of the polymer having a plurality of pendent of hydroxyl groups, at a temperature necessary to liberate free radicals for the polymerization. Typical temperatures range from 30 to 95xc2x0 C., preferably 40 and 90xc2x0 C. Reaction times typically range from 1 to 10 hours.
Examples of free radical initiators which can be employed in the graft copolymerization reaction include ammonium persulfate, sodium persulfate, potassium persulfate, and others. Redox initiation systems can also be used. Suitable reducing agents or activators include bisulfites, sulfoxylates, ascorbic acid, erythorbic acid, and other reducing sugars. Suitable oxidants include tert-butylhydroperoxide, hydrogen peroxide, and the like. Persulfate initiator systems are preferred for the poly(vinyl alcohol) grafting reactions. The amount of initiator used can be about 0.1 to 30 wt %, preferably 1 to 20 wt %, based on the amount of vinyl alcohol polymer.
Pressure during the graft polymerization reaction can be greater than about 600 psi (4238 kPa) depending on the amount of ethylene which is desired in the graft emulsion copolymer. Pressures of 1200 psi (8375 kPa) or greater can be used when the amount of initiator is 2 to 5 wt %, based on the amount of poly(vinyl alcohol), in order to produce a graft emulsion copolymer containing at least 50 wt % polyethylene.
Well known polymerization methods can be used and the reaction can be carried out as a batch or continuous reaction. A method which has been found effective in producing the graft emulsion copolymers is to first prepare the aqueous solution of the hydroxyl-containing polymer and introduce it into a reactor which has been purged with an inert gas, such as nitrogen. The ethylene and, optionally, other monomer, can then be introduced under pressure and agitation, and the temperature increased to reaction temperature. Initiator and monomers are added incrementally over the reaction period, and the reaction mixture maintained at reaction temperature for a time required to produce the desired product.
It has been found that by using specific process conditions and reactants as discussed above, a high solids, i.e., 10 to 65 wt %, preferably 30 to 60 wt %, graft emulsion copolymer product can be formed.
The product of the graft polymerization contains poly(vinyl alcohol)-g-polyethylene as well as some homo- or copolymer of the ethylene and other monomers present in the reaction mixture. The graft emulsion copolymer contains 50 to 95 wt % polyalkylene, 5 to 50 wt % hydroxyl-containing polymer, and up to 45 wt %, preferably up to 25 wt %, of other ethylenically unsaturated monomers, based on the total weight of graft emulsion copolymer.
It has also been found that the crystalline melting point of the poly(vinyl alcohol) is significantly lowered by grafting polyethylene to it; for example, the crystalline melting point can be lowered to about 100xc2x0 C. The melting point can be reduced further by copolymerizing other monomers with the ethylene.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the invention.
Airvol(copyright)502 (DPxcx9c200, 88 mol % hydrolyzed) poly(vinyl alcohol) was used in all of the examples.